The knee is composed of three bones, the patella (knee cap), the femur and the tibia. The meniscus (cartilage), composed of the lateral and medial menisci, cushion and distribute the weight of the femur uniformly across the joint. The gaps between the tibia and femur on the inside and outside of the knee are called the “medial” and “lateral” compartments, respectively. The condyle is the smooth, rounded end of the femur that allows the femur to move easily over the surface of the meniscus on the tibia. The medial and lateral femoral condyles are generally spherical with different radii, are asymmetric with respect to one another, and articulate upon the tibial plateaus. The ligaments and tendons control the motion of the knee joint. The tendons connect the patella; the ACL (anterior cruciate ligament) and PCL (posterior cruciate ligament) prevent the tibia from sliding forward or backward and limit the tibial rotation; the collateral ligaments minimize side-to-side motion and stabilize the knee.
The complicated connections of the knee joint permit the joint to have multiple degrees of freedom. The joint has three axes of rotation: flexion and extension, abduction and adduction, and internal/external rotation between the femur and the tibia. A straight leg is said to be at full extension, and during normal human gait motion the bent knee position averages about 60 degrees of flexion. The range of motion can increase with physical activity (e.g. cycling, running). As the leg undergoes flexion or extension, there is a complex asymmetric motion of the knee joint. Adduction (varus) motion occurs as the ankle moves toward the body's midline with flexion. With extension, the ankle moves away from the body's midline, resulting in abduction (valgus). In addition, the tibia internally rotates with respect to the femur as the knee flexes. As the knee extends, the tibia rotates externally. The tibia also has proximal/distal and medial/lateral motion, with respect to the femur, during flexion/extension. See U.S. Pat. Pub. Nos. 2004/0054311 and 2004/0002674, hereby incorporated by reference, for a description of knee motion as a function of extension/flexion.
Because the knee joint is used so often, and is one of the most mobile and flexible joints in the body, it is vulnerable to injury. The knee is kept in alignment by the tendons and ligaments. Malalignment of the knee joint can occur when any of the tendons or ligaments is damaged. In particular, malalignment occurs when the tibia is translated and rotated relative to the femur. Malalignment of the tibia causes abnormal loading of forces across the knee, disrupts the normal knee joint motion, and osteoarthritis (OA) of the knee. Knee OA is one of the most common orthopaedic problems with about six percent of U.S. adults over 30 years of age suffering from this disease. The total cost of knee OA was estimated as $15.5 billion in 1994. Advanced OA often requires surgery to restore leg alignment, physical function, and reduce knee joint pain.
Treatment for knee injury, particularly knee OA, often involves the combined use of knee braces and physical therapy on the knee joint, including muscle exercise. However, many prior art knee braces apply a unilateral varus force on the valgus knee or valgus force on the varus knee to relieve pressure on the side of the knee that is injured when the leg is fully extended (see U.S. Pat. No. 5,277,698). These prior art braces, called “off-loading” braces, can increase the joint space on the compressed side of the knee, thereby reducing the unbalanced force across the knee joint, when the leg is fully extended. For example, UNLOADER® (Generation II) brand knee braces reduce the load on the affected compartment by a three-point force system and a single hinge. To generate sufficient force to correct the compressive stress, off-loading braces require relatively long lever arms and strong frames. These off-loading braces, therefore, are relatively bulky, heavy, and tend to slide down the leg. These braces do not, however, compensate for the rotational malalignment of the tibia. Instead, they apply the major force only when the leg is fully extended. When the leg is fully extended, however, accessory movement of the knee is restricted (the tibia cannot rotate). Thus, any force applied to the fully extended leg is not effective in correcting the underlying rotational malalignment of the tibia.
Most braces do not properly simulate the complex three-dimensional motion of the knee joint. For example, many braces are designed to protect from external impact to the knee. Other braces only permit limited motion of the knee joint without providing for the more complicated multi-dimensional motion of the joint. U.S. Pat. No. 4,723,539 provides a knee brace hinge with a slot and slot follower permitting only an anterior/posterior motion during flexion. The center of rotation, however, remains fixed. Another type of brace hinge has linking and pivot members wherein the pivot point changes during rotational movement (U.S. Pat. No. 5,230,697), but does not control other motions. Still other braces that attempt to control knee motion in three-dimensions are bulky. See, e.g. U.S. Pat. Nos. 5,792,086 and 5,107,824. There is a need in the art to correct the rotational malalignment, as well as lateral or medial translation, of the tibia during the leg's swing phase so as to correct abnormal loading force of the knee joint as the leg nears full extension. The knee braces of the present invention do not require a non-physiological force to be applied to the knee joint, but instead restore normal knee motion by applying appropriate physiological forces during flexion and extension. Therefore, the present invention's knee braces can be relatively small and lightweight, thereby increasing the user's comfort, while restoring the normal envelope of knee motion.